Cathodic protection of steel elements at least partly embedded in a surrounding layer is well known and methods for this purpose are described in PCT Application CA00/00101 filed 2 Feb. 2000 and published as WO 00/46422 and in PCT Application CA02/00156 filed 24 Jul. 2002 and published as WO 03/010358 both by the present inventor.
In the first above application is disclosed an addition into the anode body of an enhancement material in the form of a humectant which enhances the ion flow and maintains the anode electrochemically active during its life. The enhancement material may be contained within the sacrificial anode material of the anode body rather than in a surrounding mortar or the like. In the second application is disclosed an arrangement in which the sacrificial anode material is inhibited from cracking by forming the anode material with pores which are generally sufficient to contain the expanding corrosion products. This can be achieved by compressing fine materials such as powder or flakes or by compressing or crumpling solid sheet material so that pores or interstices are formed between the crumpled layers.
In PCT Published Application WO 94/29496 of Aston Material Services Limited is provided a method for cathodically protecting reinforcing members in concrete using a sacrificial anode such as zinc or zinc alloy. In this published application and in the commercially available product arising from the application, there is provided a puck-shaped anode body which has a coupling wire attached thereto. In the commercially available product there are in fact two such wires arranged diametrically opposed on the puck and extending outwardly therefrom as a flexible connection wire for attachment to an exposed steel reinforcement member.
The puck is surrounded by an encapsulating material such as mortar which holds an electrolyte that will sustain the activity of the anode. The mortar is compatible with the concrete so that electrolytic action can occur through the mortar into and through the concrete between the anode and the steel reinforcing member.
The main feature of the published application relates to the incorporation into the mortar of a component which will maintain the pH of the electrolyte in the area surrounding the anode at a high level of the order of 12 to 14.
In use of the device, a series of the anodes is provided with the anodes connected at spaced locations to the reinforcing members. The attachment by the coupling wire is a simple wrapping of the wire around the reinforcing bar. The anodes are placed in locations adjacent to the reinforcing bars and re-covered with concrete to the required amount.
Generally this protection system is used for concrete structures which have been in place for some years sufficient for corrosion to start. In general, areas of damage where restoration is required are excavated to expose the reinforcing bars whereupon the protection devices in the form of the mortar-covered pucks are inserted into the concrete as described above and the concrete refilled.
These devices are beginning to achieve some commercial success and are presently being used in restoration processes. However improvements in operation and ergonomics are required to improve success of this product in the field.
U.S. Pat. No. 6,193,857 (Davison) assigned to Foseco discloses an anode body in the form of a puck coated with a mortar in which the puck is attached by ductile wires to the rebar within an excavation in the concrete.
In U.S. Pat. No. 5,714,045 (Lasa et al) issued Feb. 3, 1998 to Alltrista Corporation is disclosed a jacketed anode assembly for use in a sacrificial anode cathodic protection system deployed to impede corrosion of steel or steel reinforcement in pilings or similar supporting columns. A non-conductive jacket formed of mating shell halves is lined along its interior surface with sheets of expanded metal such as expanded zinc. The metal sheets are of a composition higher on the galvanic series than the steel reinforcement such that the sheets serve as sacrificial anodes when coupled with the steel reinforcement. The jacket and zinc lining are installed as a unit on the piling with the jacket interior surface facing the periphery of the piling and in spaced apart relationship therewith. In this space, a filling material can be introduced to both secure the metal sheets in place between the jacket and piling as well as serve as an electrolyte between the steel reinforcement and the metal sheet. This commercial success of this method has been limited by the high cost of the jacket which can match the cost of the active components of the anode and coating and has little technical advantage, thus providing a significant cost disadvantage.
In this arrangement the anode is located immediately at the jacket so that one purpose of the jacket is to locate and hold the anode directly at the surface of the additional cast layer. The jacket is therefore essential to this process in that it must be present in order to locate the anode. The jacket does not have sufficient structural strength to act, without additional strengthening members, as a form for the concrete and does not apply forces on the anode inside it but only acts to maintain a layer of water within the jacket which may assist in maintaining the anode active.
The anode is located directly at the surface so that it is located wholly at the surface and is not buried within the layer in a manner in which expansion of the anode member would cause breakdown or cracking of the concrete or separation of the anode from the concrete. This avoids problems of containment of the corrosion products since the corrosion products can readily diffuse away from the outside surface in the wet environment within which the columns are located, particularly as the corrosion products are formed at or adjacent the surface of the layer and the jacket retains a layer of water which can wash away the corrosion products.
Corrosion products generally occupy more volume than the anode material from which they are formed. In some cases the corrosion products are soluble or at least partially soluble so that, for example in a marine environment which is very wet or fully submerged, diffusion will occur rapidly with little or no pressure developing by the expansion of the corrosion products.
However most applications of cathodic protection systems of this type are in dry, that is non-submerged, environments and in these conditions the corrosion products are generally in solid form. Diffusion of the corrosion products in solid phase is very slow. Thus, even though the corrosion products may, over extended time periods, diffuse away from the anode body thus reducing forces from expansion, this diffusion in most cases is very slow and even where there are adjacent pores or openings in the covering material or in the anode structure itself, the corrosion products may generate significant expansion forces if the anode corrodes quickly, due to the slower diffusion movement of the corrosion products.
In most cases the galvanic anode is intended to be designed to corrode as, quickly as possible so as to maximise the protection effect. When so designed for rapid corrosion, the corrosion rate and the rate of corrosion product generation can exceed the ability of the system to absorb or carry away the corrosion products. This will therefore generate pressure due to the expansion.
It is possible to design the system so as to limit the rate of corrosion to address this problem so that the rate of diffusion or take-up of the corrosion products acts as the limiting factor to the rate of corrosion which can be accommodated. As set forth in the above published PCT application of the present inventor, increasing the porosity of the anode can increase the rate at which corrosion products can be absorbed. The same effect can be obtained by increasing the porosity of the concrete surrounding the anode body. However this can be detrimental as it decreases structural strength and provides voids which can contain water with the: potential for freeze/thaw damage.